System and Method for Preamble Detection in MIMO Narrowband Power Line Communications

ABSTRACT

A PLC network system and method operative with OFDM for generating MIMO frames with suitable preamble portions configured to provide backward compatibility with legacy PLC devices and facilitate different receiver tasks such as frame detection and symbol timing, channel estimation and automatic gain control (AGC), including robust preamble detection in the presence of impulsive noise and frequency-selective channels of the PLC network. A PLC device may include a delayed correlation detector and a cross-correlation detector operating in concert to facilitate preamble detection in one implementation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S patent application Ser. No.15/206,489, filed Jul. 11, 2016, which claims priority to U.S. patentapplication Ser. No. 14/583,875, filed Dec. 29, 2014, now U.S. Pat. No.9,419,835, which claims priority to and the benefit of U.S ProvisionalApplication Ser. No. 62/057,649, filed Sep. 30, 2014, U.S. ProvisionalApplication Ser. No. 62/057,661, filed Sep. 30, 2014, and U.S.Provisional Application Ser. No. 62/057,669, filed Sep. 30, 2014, eachof which is hereby incorporated by reference in their entirety.

FIELD OF THE DISCLOSURE

Disclosed embodiments relate generally to the field of communicationsincluding power line communications.

BACKGROUND

Power line communications (PLC) include systems for communicating dataover the same medium (i.e., a wire or conductor) that is also used totransmit electric power to residences, buildings, and other premises.Once deployed, PLC systems may enable a wide array of applications,including, for example, automatic meter reading and load control (i.e.,utility-type applications), automotive uses (e.g., charging electriccars), home automation (e.g., controlling appliances, lights, etc.),and/or computer networking (e.g., Internet access), to name a few.

Narrowband power line communications (NB-PLC) operating in the 3-500 kHzfrequency band has been gaining interest as a solution to support theemerging Smart Grid applications that aim to optimize the efficiency andreliability of the power grids. PLC is particularly appealing for SmartGrid applications due to its low deployment cost over the existing powerline infrastructure.

PLC channels are known to be highly challenging environments for digitalcommunication because they have to contend with bursts of impulse noisethat can be highly random. Further, frequency-selective channels of PLCcan significantly limit the system performance and data rates.

Multiple Input Multiple Output (MIMO) PLC is being considered as apromising technology to increase the data rates as well as providerobustness against the harsh conditions encountered in PLC environments.

SUMMARY

In MIMO PLC communications, the design of a preamble structure that canbe efficiently used to perform initial receiver synchronization is of acrucial importance. In addition, backward compatibility of the MIMOframe structure with legacy NB-PLC standards is a significant issue interms of marketability and usability.

The present patent application discloses systems, methods, devices,apparatuses and associated computer-readable media having executableprogram instructions thereon for providing or otherwise facilitatingMIMO-based data communications in a PLC network. Broadly, variousaspects of a PLC network operative with a suitable Orthogonal FrequencyDivision Multiplexing (OFDM) modulation scheme are disclosed forgenerating MIMO frames with suitable preamble portions configured toprovide backward compatibility with legacy PLC devices and facilitatedifferent receiver tasks such as frame detection and symbol timing,channel estimation and automatic gain control (AGC), including robustpreamble detection in the presence of impulsive noise andfrequency-selective channels of the PLC network.

In one aspect, an embodiment of a MIMO frame generation method operativein a PLC network including one or more MIMO PLC devices and one or morelegacy PLC devices operating according to a legacy PLC data transmissionstandard using OFDM is disclosed. The PLC network may be configured tosupport a MIMO channel having N_(T) transmit phases or ports and N_(R)receive ports. The claimed embodiment comprises, inter alia, generatingor otherwise providing at a first transmit port of a MIMO PLC device,corresponding to a first transmit phase of the MIMO frame, a legacypreamble (L-Preamble) portion compliant with the legacy PLC datatransmission standard, e.g., IEEE 1901.2 standard, which is followed bya first Frame Control Header (FCH) portion that includes legacy FCH dataand MIMO-compliant FCH data. A first MIMO-compliant preamble(M-Preamble) portion comprising a plurality of components follows theFCH portion, wherein each component includes a SYNCM symbol andassociated guard interval, collectively referred to as S₁, and a SYNCPsymbol and associated guard interval, collectively referred to as S₂. Afirst payload data portion follows the M-preamble portion as part of thefirst transmit phase. At remaining transmit ports of the MIMO PLCdevice, corresponding to the rest of the transmit phases of the MIMOframe, respective L-Preamble portions and FCH portions that arecyclic-shifted by a select amount (CS-L) are provided, which arefollowed by corresponding M-Preamble portions and respective payloaddata portions that are cyclic-shifted by a select amount (CS-M), whereinoperations at the transmit ports of the MIMO PLC device aresubstantially synchronized in time domain with respect to generating theMIMO frame.

In another aspect, an embodiment of a hybrid preamble detection methodis disclosed that is operative at a PLC device adapted to receive datain a PLC network using an OFDM modulation scheme. The claimed embodimentcomprises, inter alia, determining an initial estimate of a preamblestart in a received PLC signal stream based on a delayed correlationprocess, e.g., upon recognizing a preamble's presence therein; andresponsive to a search range around the initial estimate of the preamblestart, determining a final estimate thereof based on a cross-correlationprocess involving a known preamble sequence that is indicative of astart of a PLC frame in the received PLC signal stream.

In yet another aspect, an embodiment of a cyclic shift (CS) selectionmethod operative for a MIMO PLC network, e.g., [N_(T)×N_(R)] network, isdisclosed. The claimed embodiment comprises, inter alia, obtaining,generating or otherwise configuring an initial CS vector having defaultCS amounts that may be applied to different transmit phases of a MIMOdata signal frame at a MIMO transmitter coupled to the MIMO channel.Upon applying the initial CS vector to one or more portions of the MIMOdata signal frame, at least one of the L-Preamble and/or M-Preambleportions are transmitted over the PLC network. Thereafter, the channelis monitored by the MIMO transmitter for an Acknowledgement (ACK) frame.If no ACK signal frame is received from a receiver device within atimeout period, the initial CS vector is interleaved or otherwiserearranged in an iterative process for retransmission of the MIMO datasignal frame until the ACK signal frame is received, thereby a moreoptimal channel may be presented for data communications.

In a still further aspect, an embodiment of a collision rate reductionmethod operative for a MIMO PLC network, e.g., [N_(T)×N_(R)] network,that includes MIMO devices and legacy devices is disclosed. The claimedembodiment comprises, inter alia, obtaining, generating or otherwiseconfiguring legacy FCH data and generating a CRC sequence therefor usinga suitable generator polynomial (e.g., CRC5, CRC8, etc.). At least aportion of the legacy FCH data is intentionally perturbed to introducean error therein. The MIMO transmitter encodes and transmits the signalframe including the perturbed legacy FCH data and the CRC sequence ofunperturbed legacy FCH data, wherein the CRC sequence will automaticallyfail at a legacy PLC receiver device in order to ensure a predeterminedback-off time (e.g., a maximum back-off time) by the legacy PLC receiverdevice when it receives the MIMO data signal frame via the PLC network.In one implementation, the legacy FCH data may be perturbed so as tomaximize a Hamming distance between the perturbed and unperturbed FCHdata. In other implementations, the legacy FCH data may be disturbed byinverting all or a portion of the bits, adding extra bits, deleting oneor more bits, and the like.

In still further aspects, additional or alternative embodiments ofmethods operative at MIMO transmitter or receiver devices may beprovided in accordance with the teachings herein. In still furtherrelated aspects, embodiments of apparatuses and non-transitory tangiblecomputer-readable media containing program instructions or code portionsstored thereon are disclosed for performing one or more processes,methods and/or schemes set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure are illustrated by way of example,and not by way of limitation, in the Figures of the accompanyingdrawings in which like references indicate similar elements. It shouldbe noted that different references to “an” or “one” embodiment in thisdisclosure are not necessarily to the same embodiment, and suchreferences may mean at least one. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to effect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

The accompanying drawings are incorporated into and form a part of thespecification to illustrate one or more exemplary embodiments of thepresent disclosure. Various advantages and features of the disclosurewill be understood from the following Detailed Description taken inconnection with the appended claims and with reference to the attacheddrawing Figures in which:

FIG. 1 depicts an example PLC network wherein one or more embodiments ofthe present patent application may be practiced;

FIG. 2 depicts an example MIMO transmitter/receiver system forcommunications via a PLC channel according to an embodiment;

FIG. 3 depicts an example MIMO channel with [N_(T)=2] transmit phases orports and [N_(R)=4] receive paths operative in the embodiment of FIG. 2;

FIG. 4A depicts an example multipath OFDM transmitter system for use ina MIMO PLC system according to an embodiment;

FIG. 4B depicts an example multipath OFDM receiver system for use in aMIMO PLC system according to an embodiment;

FIG. 5A depicts an example PHY frame structure according to a legacynarrowband PLC data transmission standard using OFDM, e.g., the IEEE1901.2 standard;

FIG. 5B depicts an example PHY frame structure for multiphasetransmission in a hybrid MIMO PLC network comprising MIMO-compliant PLCdevices as well as legacy PLC devices according to an embodiment of thepresent patent application;

FIGS. 5C and 5D depict unitary space-time transmission structures for a[2×2] and a [4×4] MIMO channel, respectively, in a PLC network;

FIG. 5E depicts an example PHY frame structure for multiphasetransmission in a pure MIMO PLC network according to an embodiment;

FIG. 6A depicts an example MIMO PLC transmitter device wherein one ormore embodiments may be practiced according to the teachings of thepresent disclosure;

FIG. 6B depicts an example MIMO PLC receiver device wherein one or moreembodiments may be practiced according to the teachings of the presentdisclosure;

FIGS. 7A and 7B depict flowcharts of an example MIMO frame generationmethod according to one or more embodiments;

FIG. 8 depicts a block diagram of an example preamble detector accordingto an embodiment;

FIGS. 9A-9B and 10A-10B illustrate and exemplify a delayed correlationscheme for use with an embodiment of the preamble detector of FIG. 8;

FIGS. 11A and 11B depict example plots of delayed correlation profilesfor determining an initial estimate of the preamble start in a MIMOsignal according to an embodiment of a delayed correlation scheme;

FIG. 12 is flowchart of an example preamble detection method for usewith an embodiment of the preamble detector of FIG. 8;

FIG. 13 is flowchart of an example delayed correlation method for usewith an embodiment of the preamble detector of FIG. 8;

FIG. 14 illustrates an example cross-correlation scheme for determininga final estimate of a preamble start as part of an embodiment of thepreamble detector of FIG. 8;

FIGS. 15A and 15B depict example plots of cross-correlation profileswithout and with data peak folding according to an embodiment of thecross-correlation scheme of FIG. 14;

FIG. 16 is flowchart of an example cross-correlation method for use withan embodiment of the preamble detector of FIG. 8;

FIGS. 17A and 17B illustrate a boundary correction scheme for thepreamble symbols according to an embodiment;

FIG. 18 is flowchart of a method of symbol boundary correction accordingto an embodiment of the scheme of FIGS. 17A and 17B;

FIGS. 19A and 19B illustrate a cyclic shift (CS) selection scheme in anexample MIMO PLC network according to an embodiment;

FIG. 20 is flowchart of a CS selection method operative according to anembodiment of the scheme of FIGS. 17A and 17B;

FIG. 21 is flowchart of a collision rate reduction method according toan embodiment; and

FIG. 22 is a block diagram of an example PLC device wherein one or moreembodiments of the present patent application may be practiced.

DETAILED DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention will now be described in detailwith reference to the accompanying Figures. In the following detaileddescription of embodiments of the invention, numerous specific detailsare set forth in order to provide a more thorough understanding of theinvention. However, it will be apparent to one of ordinary skill in theart that the invention may be practiced without these specific details.In other instances, well-known features have not been described indetail to avoid unnecessarily complicating the description. As usedherein, the term “couple” or “couples” is intended to mean either anindirect or direct electrical connection unless qualified as in“communicably coupled” which may include wireless connections. Thus, ifa first device couples to a second device, that connection may bethrough a direct electrical connection, or through an indirectelectrical connection via other devices and connections.

Referring now to the drawings and more particularly to FIG. 1, depictedtherein is an example PLC network 100 wherein one or more embodiments ofthe present patent application may be practiced. For purposes of thepresent disclosure, reference numeral 102 refers to at least a portionof a power line installation comprising a plurality of power lines orwires that may include high-voltage (HV) lines (e.g., ranging from 110kV to 380 kV), medium-voltage (MV) lines (e.g., ranging from 10 kV to 30kV) and low-voltage (LV) lines (e.g., ranging from 110 V to 400 V), orany combination thereof, which may be interspersed with appropriategrid-to-customer infrastructural components such as substations,transformers, phase converters, relays, coupling capacitors, electricityusage meters, breaker panels and other elements facilitating powerdistribution over varying distances (e.g., ranging from hundreds ofmiles to distances within customer premises). Accordingly, power lineinstallation 102 may be localized within single-family residences,apartment buildings, industrial/commercial premises, etc., as well asextending beyond in certain embodiments, wherein the variousinfrastructural components are not shown for the sake of clarity. In oneillustrative implementation, power line installation 102 may be providedas part of a power distribution system disclosed in commonly-assignedU.S. Pat. No. 8,265,197 entitled “OFDM TRANSMISSION METHODS IN THREEPHASE MODES,” issued in the name(s) of II Han Kim et al., incorporatedby reference herein.

A plurality of electrical appliances 104-1 to 104-N, which may compriseany known or heretofore unknown residential, industrial or commercialappliances, may be coupled to the power line installation 102 in aconventional manner. Whereas appliances 104-1 to 104-N do not normallytake part in data communications over the wiring of power lineinstallation 102, they can affect the channel over which PLC datadevices must communicate. In an alternating current (AC) implementation,depending on whether single-phase power or polyphase (e.g., three-phase)power is being distributed, the wiring of power line installation 102may comprise a plurality of conductors, which often vary based on acountry's electrical transmission standards. In a typical U.S. homeinstallation, three wires, i.e., a phase (P) or live (L) line, a neutral(N) line and a protective earth (PE) line, may be provided. For athree-phase transmission system, three separate live lines (i.e., L1, L1and L3) may be provided in addition to the N and PE lines.

In a backward-compatible implementation of the example PLC network 100,a plurality of Single-Input Single-Output (SISO) modems or data devices108-1 to 108-M may be coupled to a pair of the wires of power lineinstallation 102, e.g., the P-N wire pairing for both transmit andreceive devices, for communicating data according to a legacy PLC datatransmission standard using OFDM, e.g., PRIME, G3, ITU G.hnem, Home PlugAV, IEEE 1901.2, and the like. Further, a plurality of Multiple-InputMultiple-Output (MIMO) modems or data devices 106-1 to 106-K may beadvantageously coupled to two or more wiring pairs of example power lineinstallation 102 for communicating data according to the embodiments setforth in detail hereinbelow.

FIG. 2 depicts an example MIMO transmitter/receiver system 200 foreffectuating data communications via a PLC channel 202 according to anembodiment wherein a 3-wire power line installation is illustrated. Atransmit (Tx) modem or data device 204 and a receive (Rx) modem or datadevice 206 are coupled to the 3-wire power line installation comprisingL line 208-1, N line 208-2 and PE line 208-3. On the transmit side,although data signals may be fed using the three pairs of wiring, i.e.,P-N, P-PE and N-PE pairs, because of Kirchoff's law that the sum of thethree input signals must be zero, only two of the three pairs may beused as transmit ports for data injection. On the receive side, allthree P-N, P-PE and N-PE pairs may be used as receive ports in additionto a common mode (CM) signal, which is the voltage difference betweenthe sum of the voltages on the three wires and the ground, shown in FIG.2 as CM port 208-4. It should be appreciated by one skilled in the artthat for EMI reasons, the CM path may be used for the receiving sideonly.

FIG. 3 depicts an example MIMO channel 300 with two transmit ports andfour receive ports operative in the embodiment of FIG. 2. Referencenumerals 302-1 and 302-2 refer to two example transmit ports, L-N andL-PE, respectively, which as will be seen further below can give rise totwo corresponding transmit phases in a MIMO transaction. Accordingly,the terms “transmit port” and “transmit phase” may be used somewhatinterchangeably in certain embodiments of the present patentapplication. On the receive side, data may be received via all fourreceive ports, L-N 304-1, L-PE 304-2, N-PE 304-3 and CM 304-4, which maybe referred to as receive paths. Accordingly, MIMO channel 300 isconfigured as a [2×4] channel in an exemplary 3-wire power lineinstallation of FIG. 2. Similar to the [2×4] MIMO channel 300exemplified herein, a generalized MIMO channel of [N_(T)×N_(R)] may beformed in a power line installation having a plurality of wiresdepending on the particular combinations of wire pairings being used. Itshould be appreciated that not all possible combinations of ports on thetransmit side or receive side may need to be utilized in a MIMO PLCchannel. Further, although the foregoing MIMO channel embodiments arebased on AC power lines, a DC transmission system may also be used forimplementing a MIMO channel. One skilled in the art will recognize thatthe return path may not be a ground line in certain DC transmissionsystems. In such a scenario, a MIMO channel may be implemented, forexample, by using the earth as ground with respect to the energizedlines of a DC transmission system, which facilitates multiple Tx/Rxports via appropriate differential pairs.

A suitable OFDM technique may be utilized for transmitting data in anexample MIMO PLC network described above. Referring now to FIGS. 4A and4B, high level block diagrams of example multipath OFDM transmitter andreceiver systems are respectively illustrated, which may be implementedas part of a MIMO modem of a PLC device operative to effectuate Tx andRx operations using suitable PHY level MIMO data frames as will bedescribed in detail further below. In the following sections of detaileddescription, when the focus is on Tx operations at a PLC device, such adevice or modem may be referred to as a PLC transmit device or modem.Likewise, a PLC receive device or modem may be described in particularreference to Rx operations.

At transmit device 400A, incoming data (Data In) 402 may beappropriately encoded involving a forward error correction (FEC) block404, which may then be provided to a serial-to-parallel (S/P) converter406 for transmission via a number of transmit paths or ports. On eachpath, a suitable modulator, e.g., adaptive quadrature amplitudemodulation (QAM) or phase-shift keying (PSK), is operative to map theencoded data streams into symbols, as illustrated by modulator blocks408-1 to 408-N. A MIMO encoder block 410, which may be based onbeamforming, spatial multiplexing or space-time block coding, etc., isoperative to generate data streams of suitable diversity that areprovided to respective inverse fast Fourier transform (IFFT) blocks412-1 to 412-N. As will be described below, the output of IFFT blocks412-1 to 412-N may be further processed and provided as a MIMO datasignal comprising multiphase PHY level data frames that are injectedinto a power line installation via a front end block operative toeffectuate a corresponding number of Tx paths 414-1 to 414-N. A feedbackblock 416 may be configured to provide various pieces of informationfrom a receiver device (e.g., relative to channel estimation or quality,adaptive OFDM tone map information, acknowledgement signals, etc.) tothe receiver 400A, at least part of which may be used in effectuatingcertain embodiments set forth herein.

At receiver 400B, a plurality of FFT demodulators 454-1 to 454-K areprovided corresponding to K receive paths 452-1 to 452-K that may beeffectuated via a suitable front end block (not shown). A channelequalizer and associated channel estimation module 456 may be used tocalculate a suitable channel matrix that may be used by a decoder 458 toobtain or recover a plurality of data paths, which are demodulated bycorresponding demodulators 460-1 to 460-N. Respective data streams areprovided to a parallel-to-serial (P/S) converter 462 for assembling thedata. Thereafter, FEC decoding 464 is applied to obtain the data 466. Asillustrated, channel estimation block 456 may provide appropriateinformation to MIMO decoding and demodulation blocks as exemplified bysignal paths 470, 474. In addition, channel information may be providedas feedback information to transmitter 400A. Additional details relativeto OFDM transmission in a MIMO channel using beamforming and othertechniques may be found in U.S. Pat. No. 8,265,197, referenced andincorporated hereinabove.

As pointed out previously, PLC data communications using OFDM may beaccomplished according to a variety of standards including, e.g., IEEE1901.2. Without limitation, an overview of IEEE 1901.2 will now bedescribed in order to better understand certain embodiments as exampleimplementations of the present invention. A more detailed overview isprovided in “An Overview, History, and Formation of IEEE P1901.2 forNarrowband OFDM PLC”, Jul. 2, 2013. Both this document and IEEE 1901.2specification entitled “Standard for Low-frequency (less than 500 kHz)Narrowband Power Line Communications for Smart Grid Applications”,approved Oct. 31, 2013, are incorporated by reference herein.

IEEE 1901.2 specifies communications for low frequency (less than 500kHz) narrowband power line devices via AC and DC electric power lines.Broadly, it describes Physical (PHY) layer and Media Access Control(MAC) modes of operation for Federal Communications Commission (FCC),European Committee for Electrotechnical Standardization (CENELEC), andAssociation of Radio Industries and Businesses (ARIB) bands. Thisstandard supports indoor and outdoor communications in the followingenvironments: a) low voltage lines (less than 1000 V), such as the linebetween a utility transformer and meter; b) through transformerlow-voltage to medium-voltage (1000 V up to 72 kV); and c) throughtransformer medium-voltage to low-voltage power lines in both urban andin long distance (multi-kilometer) rural communications. The standarduses transmission frequencies less than 500 kHz and data rates may beconfigured to be scalable to 500 kbps depending on the applicationrequirements. This standard addresses grid-to-utility meter, electricvehicle to charging station, and within home area networkingcommunications scenarios, among others.

The MAC layer is an interface between the logical link control (LLC)layer and the PHY layer. The channel access is accomplished by using thecarrier sense multiple access with collision avoidance (CSMA/CA)mechanism with a random back-off time. The random back-off mechanismspreads the time over which stations attempt to transmit, therebyreducing the probability of collision. Each time a device wishes totransmit data frames, it waits for a random period. If the channel isfound to be idle, following the random back-off, the device transmitsits data. If the channel is found to be busy, following the randomback-off, the device waits for another random period before trying toaccess the channel again.

Details regarding PHY building blocks have been standardized in variousIEEE publications, resulting in a uniform PHY structure for NB PLC. Asis known in the art, the fundamental PHY elements in the transceiverstart with a scrambler operating as part of a FEC block, which functionsto randomize the incoming data. Standardized scramblers (e.g., in IEEE1901.2, G3-PLC and PRIME) utilize the same generator polynomial, asillustrated in the following equation:

S(x)=x ⁷ ⊕x ⁴⊕1

Two levels of error correction may follow, starting with a Reed-Solomon(RS) encoder where typically data from the scrambler is encoded byshortened systematic Reed-Solomon (RS) codes using Galois Field (GF).The second level of error correction may employ a convolutional encoderwith constraint rate K=7, which is followed by a two-dimensional (timeand frequency) interleaver. These blocks may be configured tointeroperate together in order to improve robustness and overall systemperformance in the presence of noise.

Following the FEC is the OFDM modulator involving one or more variantsof modulation (BPSK, QPSK, 8PSK, etc.). The defined modulator furtherdescribes constellation mapping; the number of repetitions (4, 6, etc.);the type of modulation (differential, coherent); the frequency domainpre-emphasis; OFDM generation (IFFT, with cyclic prefix or CP); andwindowing.

Structure of the physical frames is defined according to the fundamentalsystem parameters, including the number of FFT sample points (e.g., 256,512, etc.) and overlapped samples, the size of cyclic prefixes, thenumber of symbols in the preamble, and the sampling frequency. The PHYlayer supports two types of frames: the data frame and the ACK/NACKframe. Each frame starts with a preamble used for synchronization anddetection, as well as automatic gain control (AGC) adaptation. Thepreamble is followed by data symbols allocated to the Frame ControlHeader (FCH) with the number of symbols depending on the number ofcarriers used by the OFDM modulation.

The FCH is a data structure transmitted at the beginning of each dataframe following the preamble. It contains information regardingmodulation and the length of the current frame in symbols. The FCH alsoincludes a frame control checksum (CRC, or cyclic redundancy check),which is used for error detection. The size of the CRC depends on thefrequency band being utilized (e.g., CRC5 for CENELEC A and CENELEC Band CRC8 for FCC).

A typical legacy frame structure of a PHY data frame 500A according toIEEE 1901.2 for SISO data transmission is shown in FIG. 5A. By way ofillustration, data frame 500A comprises a legacy preamble (L-Preamble)portion 502 that may be composed of 8 or 12 identical synchronizationplus 1 (SYNCP) symbols and one full synchronization minus 1 (SYNCM)symbol and the first half of a SYNCM symbol in the time domain. Each ofthe SYNCP and SYNCM symbols is provided to be 256 samples long. TheL-Preamble portion 502 is immediately followed by FCH portion 504. TheSYNCP symbols are used for AGC adaptation, symbol synchronization,channel estimation, and initial phase reference estimation. The SYNCMsymbols are identical to SYNCP symbols except that all the carriers areπ phase shifted. At the receiver, the phase distance between symbolSYNCP and symbol SYNCM waveforms may be used for frame synchronization.Typically, a SYNCP symbol is generated by creating the desired number ofequally spaced active carriers, with a magnitude of one. The phase ofeach carrier given by ϕ_(c) is a multiple of π/8 and is obtainedstarting with a chirp-like sequence carrier phases over the desiredbandwidth. Thus, a possible method to generate the SYNCP symbol is tostart in the frequency domain and create 72 complex carriers with theinitial phases ϕ_(c).

The data symbols immediately after the L-Preamble portion 502 arereserved for FCH 504, the number of which may vary depending upon theband plan in use. For example, the FCH length is 12 symbols when 72tones are used and no tone mask is applied. The FCH may be coherentlymodulated and a Coherent Mode bit in the FCH may be used to indicatewhether differential mode or optional coherent mode of operation isindicated. The differential mode uses differential modulation schemesthat do not require channel estimation to demodulate the receivedsymbols (after initial channel estimation operations), while thecoherent mode uses coherent modulation schemes that requires channelestimation be done in a more adaptive manner. FCH portion 504 containsinformation regarding the current frame, e.g., the type of frame, thetone map index of the frame, the length of the frame, etc. which is usedby legacy devices. A coherent mode (CM) portion 506 comprising S₁ and S₂symbols 506A, 506B, which are SYNCM and SYNCP symbols, respectively, mayimmediately follow FCH portion 504. The S₁ and S₂ symbols 506A, 506B aretransmitted only in the coherent mode in order to be used for performinginitial channel estimation for the data demodulation.

FIG. 5B depicts an example PHY frame structure 500B for multiphasetransmission in a hybrid MIMO PLC network comprising MIMO-compliant PLCdevices as well as legacy PLC devices according to an embodiment of thepresent patent application. It should be appreciated that the framestructure 500B is operative for providing backward compatibility withthe legacy frame structure set forth in FIG. 5A, and may be generalizedfor a MIMO channel configured with N_(T) transport phases/ports andN_(R) receive paths. Additionally or alternatively, as will be seenbelow, it may be further generalized for a pure MIMO PLC network whereinno legacy devices (i.e., SISO devices) are deployed. By way ofillustration, the backward-compatible MIMO frame structure 500B isadapted for a channel configuration with N_(T) ports, wherein N_(T)phases 512-1 to 512-N_(T) of the frame are generated at corresponding Txports of a MIMO PLC device in a substantially synchronized manner in thetime domain. To facilitate backward compatibility, the MIMO framestructure 500B starts with a legacy preamble (L-Preamble) portionaccording to a legacy PLC data transmission standard (e.g., IEEE 1901.2)so that legacy SISO (or “single-phase”) devices can still detect thepreamble of a MIMO frame and avoid transmitting data during an ongoingMIMO packet transaction. Following the L-Preamble portion, an FCHportion is provided that includes both legacy FCH data (i.e.,information needed by the legacy devices) and MIMO-compliant data (i.e.,information needed by the MIMO devices). A MIMO-compliant preamble(M-Preamble) portion follows the FCH portion, wherein S₁ and S₂ symbols(i.e., SYNCM symbol plus its guard interval (GI) and SYNCP symbol plusits GI) are advantageously provided, in a repetitive manner, for channelestimation required for the payload demodulation based on the number ofnumber of Tx ports. For instance, each pair of S₁ and S₂ symbols may betreated as a component (P, or interchangeably P) wherein the number ofcomponents corresponds to the number of Tx ports, thereby resulting inN_(T) components per transmit phase in the M-Preamble portion of theMIMO frame 500B, which is followed by a corresponding payload dataportion.

Accordingly, a first transmit phase, e.g., phase 512-1, comprises afirst L-Preamble portion 514-1, followed by a first FCH portion 516-1,which in turn is followed by a first M-Preamble portion 518-1 comprisinga plurality of components, i.e., multiple repetitions of P={S₁:S₂}, anda first payload data portion 520-1. Likewise, each of the remainingphases 512-2 to 512-N_(T) comprises corresponding portions asillustrated in FIG. 5B. Further, in accordance the teachings herein,select amounts of cyclic shift (CS) may be applied to one or more frameportions of the remaining transmit phases in order to avoid undesirableeffects, e.g., unintended beamforming, etc. For example, transmittingthe same L-Preamble sequence over all the transmit phases might resultin receiving a null if a destructive combining occurs over the MIMOchannel. Accordingly, in order to avoid such consequences, different CSvalues may be applied to the L-Preamble sequence transmitted from thedifferent power phases. Also, the same CS values may be applied to theFCH symbols. It should be appreciated that the CS amount (CS-L, orinterchangeably cs-I or cI) used for the L-Preamble may be configured tobe sufficiently small in order to ensure that the legacy devices canstill be able to detect the preamble. In addition, a cyclic shift (CS-M,or interchangeably cs-m or cm) may also be applied to the M-Preamble andthe payload portions, which may be configured to be sufficiently largeto minimize the power fluctuation between the M-Preamble and the payloadto allow for proper automatic gain control and avoid signal clipping.Accordingly, the CS amounts applied to the L-Preamble and the M-Preambleportions may be configured to be different since they have differentdesign criteria. One skilled in the art will recognize that the CSamounts may be obtained through channel models, simulations and/ormeasurements. For example, CS amounts may comprise integer multiples ofa sample length and may be applied on a symbol-by-symbol basis similarto the OFDM cyclic prefix/suffix insertion.

Continuing to refer to FIG. 5B, it can be seen that the L-Preambleportions 514-2 to 514-N_(T) of remaining phases 512-2 to 512-N_(T) arecyclic-shifted by varying CS-L amounts, e.g., L-Preamble 514-2 having aCS amount [cI], L-Preamble 514-3 having a CS amount of [cI2], and so on.The same CS amounts are also applied to the FCH portions 516-2 to516-N_(T). The M-Preamble portions 518-2 to 518-N_(T) are likewisecyclic-shifted by varying CS-M amounts, [cm] to [cm−(N_(T)−1)].Additionally and/or alternatively, the same CS-M amounts may be appliedto the payload data portions 520-2 to 520-N_(T). Furthermore, the S₁ andS₂ symbols of one or more of the {P} components of the M-Preambleportions of the remaining phases may be further manipulated, e.g.,negation, time-inversion, etc., in order to achieve a unitary space-timediversity that can minimize noise enhancement during the MIMO channelestimation. It should be appreciated that such a preamble structure mayoperate to ensure substantially equalized power allocation on alltransmit phases in a way not to detrimentally affect the channelquality. In the example embodiment of FIG. 5B, M-Preamble 518-2 oftransmit phase 512-2 is shown to comprise components {−P} and {P},thereby indicating a phase shifting of the first component. Examples ofunitary transmission matrix structures 500C, 500D for [2×2] and [4×4]MIMO channels with orthogonal designs are shown in FIGS. 5C and 5D,respectively. In these embodiments, the X-axis represents thetransmission time slot and the Y-axis represents the transmit powerphase corresponding to a transmit port. In the coherent mode, theL-Preamble may be used to provide channel estimation for decoding theFCH, while the M-Preamble may be used to provide automatic gain controland channel estimation and for payload demodulation.

FIG. 5E depicts an example PHY frame structure 500E for multiphasetransmission in a pure MIMO PLC network according to one embodiment. Asthere is no need for the frames to be backward-compatible in a MIMO-onlyPLC implementation, the example frame structure 500E does not includeL-Preamble portions. Each transmit phase therefore begins with aMIMO-compliant preamble (i.e., M-Preamble) portion that includes aplurality of {P} components, for example, corresponding up to N_(T) Txports, wherein each {P} component includes the S₁ and S₂ symbolsdescribed hereinabove. An FCH portion thereafter follows which nowincludes data that is needed by the MIMO-compliant devices only. Apayload portion then follows similar to a legacy data frame structure.Whereas a first transmit phase 552-1 may comprise frame portions thatare not cyclic-shifted, e.g., M-Preamble 554-1, FCH portion 556-1 andpayload portion 558-1, the remaining phases may include frame portionshaving varying CS amounts, e.g., [cm] to [cm−(N_(T)−1)], similar to thetreatment set forth above. Accordingly, M-Preamble portions 554-2 to554-N_(T), FCH portions 556-2 to 556-N_(T) and payload portions 558-2 to558-N_(T) may include suitable amounts of CS, which may be applied asbefore, e.g., on a symbol-by-symbol basis similar to OFDM cyclicprefix/suffix insertion. Further, the CS amounts may be varied ininteger multiples of a sample length in one example implementation.Similar MIMO preamble designs may be obtained for a more generalized[N_(T)×N_(R)] PLC architecture as well.

FIG. 6A depicts a block diagram of an example OFDM transmitter deviceoperative to generate MIMO frames for MIMO PLC operation according toone or more embodiments of the present disclosure. In abackward-compatible PLC implementation, legacy FCH data 612 may beprovided for use with legacy device operations as discussed previously,which may be processed in a frame control encoder block 628 comprising ascrambler 630 and encoder and CRC generator 632. In addition, a dataperturbation block 629 may be optionally provided for purposes ofintroducing intentional errors in the FCH data in order to force a CRCfailure at a legacy receiver in certain embodiments discussed furtherbelow. Another frame control encoder block 622 may be provided forprocessing MIMO-compliant FCH data 610, for example, including scrambler624 and encoder and CRC operations 626. Payload data 608 likewise passesthrough an encoder block 614 that may include scrambler 616, convolutionencoder 618 and channel and robust OFDM interleaver 620. A MIMO transmitencoder block 634 is operative to receive the outputs from FCH encoders628, 622 and payload encoder 614. A stream mux block 636 of the MIMOtransmit encoder block 634 is operative to combine the encoded legacyFCH data, encoded MIMO FCH data and encoded payload into a plurality(N_(T)) of transmit data streams corresponding to (N_(T)) phases of aMIMO frame. The data streams are provided to a multi-channel OFDMmodulator 635 comprising corresponding symbol mapping blocks 638-1 to638-N_(T) that are coupled to a power allocation and precoding block640, which in turn is coupled to corresponding IFFT blocks 642-1 to642-N_(T). Each modulated steam is processed by cyclic prefix, windowingand overlapping blocks 644-1 to 644-N_(T) according to applicable OFDMschemes. A preamble insertion block 646 is operative to generateappropriate L-Preamble and M-Preamble portions as necessary, along withsuitable CS selection 648, for the transmit phases as discussedpreviously. An analog front end (AFE) block 604 having an appropriatenumber of ports (e.g., up to N_(T)) couples the multiphase signal to apower line installation 602 of the PLC network as exemplified by paths606-1 to 606-N_(T). Furthermore, CS selection 648 and/or preambleinsertion block 646 may be configured to receive feedback 650 fromreceiver devices to effectuate additional functionalities according tocertain further embodiments described below.

FIG. 6B depicts a block diagram of an example OFDM receiver 600B whereinone or more embodiments may be practiced according to the teachings ofthe present disclosure. A MIMO AFE block 662 may be coupled to the powerline installation 602 via a plurality of receive ports 606-1 to606-N_(R) and is operative with corresponding AGC modules 664 and timesynchronization blocks 666 to feed separate FCH and payload recoverymodules. A preamble detection block 667 may be provided in associationwith the time synchronization block(s) 666 to detect preambles andsymbol boundaries in a received MIMO signal that includes varying CSamounts across different transmit phases, as will be described inadditional detail further below.

In general, data recovery processes at receiver 600B are roughlyopposite of the operations at transmitter 600A, e.g., in reverse order,and will not be described in additional detail herein. Broadly, a legacyFCH decoder block 678 including suitable FFT modules is operative torecover legacy FCH data 684. MIMO payload and FCH data may be processedthrough another suitable number of FFT modules 668-1 to 668-N_(R), whichfeed into a MIMO equalizer 672 that generates or otherwise recovers thetransmit streams which are demodulated by corresponding demodulatorblocks 674-1 to 674-N_(T). A MIMO demux block 676 is operative toprocess the demodulated streams, feeding into a payload data decoder 682for recovering payload data 688. MIMO-compliant FCH data 686 may also berecovered from the demux output, which is separately processed by a MIMOFCH decoder 680.

FIGS. 7A and 7B depicts flowcharts of an example MIMO frame generationmethod according to one or more embodiments, depending on whetherbackward-compatible MIMO frames or pure MIMO-only frames are involved.One skilled in the art will recognize upon reference hereto that varioussteps, blocks or acts set forth in the two flowcharts may be combined ina complementary fashion in a number of ways. Reference numeral 700A ofthe flowchart in FIG. 7A illustrates the operations that may take placeat MIMO transmitter (i.e., a PLC device having a MIMO modem). By way ofgeneralization, the MIMO transmitter may be configured to operate with aMIMO PLC channel having N_(T) transport ports/phases, and N_(R) receiveports. As described above, the MIMO PLC channel is adapted to transferdata using a suitable OFDM modulation scheme. At block 702, thetransmitter is operative to generate or otherwise provide at a firsttransmit port, a first MIMO-compliant preamble portion (M-Preamble-1)comprising a plurality of components (e.g., N_(T) components), eachincluding a SYNCM (S₁) and a SYNCP (S₂) symbol having corresponding GIand/or CP. The transmitter is further configured to provide or otherwisegenerate a first MIMO-compliant Frame Control Header following theM-Preamble-1 portion (block 704). Thereafter, a first payload dataportion (PLD-1) is provided following the FCH-M portion (block 706). Ata next transmit port, e.g., a second transmit port, a secondM-Preamble-2 portion comprising a plurality of components (e.g., N_(T)components) is generated or otherwise provided (block 708). As discussedabove, each component includes S₁ and S₂ symbols that are cyclic-shiftedby a select amount (CS-M), wherein S₁ and S₂ symbols of one or more ofthe components are inverted (e.g., multiplied by −1). As set forth atblocks 710, 712, a second MIMO-compliant FCH portion following theM-Preamble-2 portion is provided, wherein the second MIMO-compliant FCHportion is a cyclic-shifted version of the first MIMO-compliant FCHportion. Thereafter, a second payload data portion (PLD-2) following thesecond FCH-M portion is provided or otherwise generated. At remainingtransmit ports, actions relating to generating or otherwise providingappropriate MIMO-compliant preamble portions, followed by MIMO-compliantFCH portions that are followed by respective payload data portions, areperformed (block 714). These frame portions are cyclic-shifted bycorresponding amounts of CS-M equaling integer multiples of a selectnumber of IFFT samples of a symbol in one implementation. It should beapparent that the foregoing operations at transmit ports of thetransmitter are substantially synchronized in time domain with respectto generating a MIMO frame.

In a hybrid or mixed-mode MIMO PLC network (e.g., including one or moreMIMO-compliant devices and one or more legacy SISO devices), process700B shown in FIG. 7B illustrates the various steps, blocks and/or actsthat may take place a backward-compatible MIMO transmitter. As before,the MIMO transmitter may be configured to operate in a MIMO PLC channelhaving N_(T) transport ports/phases and N_(R) receive ports. At block752, the frame generation process involves generating or otherwiseproviding at a first transmit port a legacy preamble portion(L-Preamble), e.g., compliant with IEEE 1901.2 standard, and an FCHportion including a legacy FCH data and MIMO-compliant FCH data,followed by a MIMO-compliant preamble portion (M-Preamble-1) comprisinga plurality of components (e.g., N_(T) components), each including S₁and S₂ symbols, which is followed by a first payload data portion. Atblock 754, the frame generation process involves, generating orotherwise providing at remaining ports corresponding L-Preamble portionsand FCH portions that are cyclic-shifted by an amount (CS-L) or integermultiples thereof, followed by corresponding M-Preamble portions thatare followed by respective payload data portions, which arecyclic-shifted by corresponding amounts of CS-M. As before, operationsat transmit ports are substantially synchronized in the time domain withrespect to generating a backward-compatible MIMO frame. It should beappreciated that in a mixed-mode MIMO PLC network implementation, it ispossible to effectuate transactions between a MIMO device and a SISOdevice in either directions, i.e., receive and/or transmit operations.

As pointed out previously, it is necessary for a receiver to synchronizetime and detect preambles properly in a received PLC signal so that datacan be recovered without errors. This is particularly relevant where arobust detection performance is required in the presence of impulsivenoise and frequency-selective channels that afflict most PLCenvironments.

Preamble detection according to the teachings herein advantageouslyexploits the repetitions in a preamble portion (e.g., legacy and/or MIMOpreamble portions) to perform delayed correlation and/orcross-correlation operations in a hybrid detection scheme as will be setforth below. The delayed correlation involves correlating the receivedstream with a delayed version of itself, while the cross-correlationinvolves correlating the received stream with the known preamblesequence. A delayed correlation detector may be implemented relativelyeasily through recursive computations (i.e., low implementationcomplexity). In addition, the delayed correlation detector performanceis typically not affected by the cyclic shift applied to the preamblesequence, as the repetitions within the preamble remain identical afterapplying the cyclic shift. On the other hand, a cross-correlationdetector has a better performance than the delayed correlation detectorbut requires a higher degree of computational complexity. Further, across-correlation detector may experience some performance degradationdue to the presence of non-zero CS amounts in a preamble sequence. Itshould be appreciated that in such a scenario multiple peaks mightappear at different locations, spaced according to the relative CSvalues across the transmit phases, instead of a single peak in the caseof zero cyclic shift. The additional peaks can give rise to adegradation of the detection signal-to-noise ratio (SNR), which canadversely affect the detection performance.

FIG. 8 depicts a block diagram of an example preamble detector 800according to an embodiment that involves a hybrid detection schemewherein both delayed correlation and cross-correlation detectors areemployed. It should be appreciated that the hybrid preamble detectiontechnique disclosed herein provides flexibility in obtaining thedesirable tradeoff between performance and complexity. Broadly, adelayed correlation detector 804 is used as the first detectionphase/stage to provide a rough estimate for the preamble start that canbe verified and refined through a second detection phase/stage thatadopts a cross-correlation detector 812. Accordingly, the hybriddetection scheme of the present disclosure takes advantage of therelatively low degree of complexity of the delayed correlation detectorand its robustness to non-zero cyclic shifts while exploiting the robustdetection performance of the cross-correlation detector to impulsivenoise and frequency-selective channel conditions of a PLC network. Asshown in FIG. 8, the delayed correlation detector 804 is configured toprocess a received signal 802, preferably in its entirety, to decide onthe presence/absence of a preamble, and find a rough estimate of thepreamble start 806, if the presence of a preamble is detected.Thereafter, in case of preamble detection, the rough estimate ofpreamble start 806 along with a certain search range 808 around theestimated preamble start are used to specify a search window 810 that isfed as the input signal to a cross-correlation detector 812 thatreconsiders the decision taken by the delayed correlation detector 804and refines the initial/rough estimate of the preamble start 806. Thesearch range of the second stage of the hybrid scheme is a key parameterthat characterizes the performance/complexity tradeoff of the hybridcorrelation scheme. Specifically, the higher the search range, thebetter the detection performance and the higher the complexity.

Taking reference to FIG. 12 in conjunction herewith, an embodiment of ahybrid detection process 1200 sets forth the steps, acts and/or blocks1200 that may take place at a PLC receiver, e.g., a MIMO device or aSISO device, for detecting either a legacy preamble sequence or a MIMOpreamble sequence in a PLC data signal. At block 1202, delayedcorrelation is applied as a first detection phase to obtain or otherwisedetermine a rough estimate of a preamble start in the received PLCsignal (i.e., an initial estimate), provided a preamble is determined tobe present. At block 1204, a search window around the rough estimatedpreamble start is applied in a second detection phase using across-correlation scheme to obtain or otherwise determine a fine-grainestimate of the preamble start in the received PLC signal (i.e., finalestimate) that is indicative of a frame. It should be appreciated thatthe hybrid preamble detector 800 and associated detection process 1200may be advantageously implemented in a PLC device such as receiver 600Bdescribed hereinabove.

Additional details regarding embodiments of a delayed correlationdetector and a cross-correlation detector operative in the hybridpreamble detector 800 are set forth immediately below.

FIGS. 9A-9B and 10A-10B illustrate and exemplify a delayed correlationdetection scheme. Example plots of delayed correlation profiles fordetermining an initial estimate of the preamble start in a MIMO signalhaving a preamble with known number of symbols are shown in FIGS.11A-11B. A flowchart of an example delayed correlation method for usewith an embodiment of the preamble detector of FIG. 8 is shown in FIG.13.

Reference numeral 900A in FIG. 9A refers to a sliding window scheme usedin computing delayed correlation with respect to a received signal 902having a preamble of 9 symbols. As shown, a sliding window 904 of 9symbols (spanning 8 SYNCP symbols and one full SYNCM symbol) slides overthe received stream 902 one sample at a time. For example, referencenumeral 906 refers to the window that has been moved by one sample.Within each window placement, each symbol is correlated with a specifiednumber (N_(C)) of symbols immediately preceding it (i.e., itscorrelation order). By way of illustration, reference numerals 908-1 to908-N_(C) refer to correlation orders 1, 2, . . . , N_(C). The totalnumber of delayed correlations per window placement, denoted as L, canbe calculated according to the below equation:

$L = {{\left( {K - 1} \right)N_{C}} - {\sum\limits_{j = 1}^{N_{C} - 1}j}}$

where K is the number of symbols within the sliding window, which is setto 9 in the illustrative example. The absolute values of the L delayedcorrelations are added together to compute the total correlation valuecorresponding to the current window placement. FIG. 9B depicts a circuitblock 900B that applies absolute value functions 922-1 to 922-L to Lcorrelations 920-1 to 920-L obtained per window placement, which areprovided to a summation block 924 for generating a total correlationvalue 926.

A simplified example for illustrating the calculation of a delayedcorrelation profile is depicted in FIGS. 10A-10B. In this example, thenumber of symbols within a sliding window (outer window) 1002A is set to4 symbols W₁ to W₄, the numbers of samples per symbol (inner window) isset to 3 samples X₁ to X₃, and the correlation order, N_(C), is set to2. Hence, the total number of correlations per window placement, L, canbe calculated to be 5. As shown in FIG. 10A, in the first step, eachsymbol, except for the first and second symbols (the 1^(st) symbol beingcorrelated with none and the 2^(nd) symbol being correlated with onlyone preceding symbol), is correlated with its two preceding symbols(e.g., on a sample by sample basis) and the absolute values of theresulting correlations are added together to obtain D₁, which is thetotal delayed correlation 1006A corresponding to the first windowplacement. The correlation equations for L=5 correlations possible(i.e., W₄ to W₃; W₄ to W₂; W₃ to W₂; W₃ to W₁; and W₂ to W₁) and thetotal correlation D₁ are set forth below:

$C_{1} = {\sum\limits_{j = 1}^{3}{W_{2j}W_{1j}}}$$C_{2} = {\sum\limits_{j = 1}^{3}{W_{3j}W_{2j}}}$$C_{3} = {\sum\limits_{j = 1}^{3}{W_{3_{j}}W_{1_{j}}}}$$C_{4} = {\sum\limits_{j = 1}^{3}{W_{4_{j}}W_{3_{j}}}}$$C_{5} = {\sum\limits_{j = 1}^{3}{W_{4j}W_{2j}}}$$D_{1} = {\sum\limits_{i = 1}^{L}{C_{i}}}$

In the second step shown in FIG. 10B, the window 1002B slides over thereceived stream with one sample (X₁₃) 1008 inserted and one sample (X₁)dropped, and the same calculations may be performed to produce D₂, whichis the total correlation 1006B for the second window placement, ascalculated according to the following equation:

$D_{2} = {\sum\limits_{i = 1}^{L}{C_{i}}}$

This processing is repeated for each window placement to obtain adelayed correlation profile over the received stream. An example of thedelayed correlation profile 1100A is shown in FIG. 11A, wherein theX-axis represents the sample index and the Y-axis represents correlationvalues, with various horizontal lines representing different thresholdlevels. As depicted in FIG. 11B, a select threshold 1110 is applied overthe correlation output and the preamble start is selected to be theindex of where a maximum correlation occurs (e.g., maxima 1116) over awindow of length that is defined by a maximization length parameter 1114and starts at the index of the first upward threshold crossing 1112.That is, the first upward threshold crossing 1112 defines the beginningof a window within which a local maximum may be found.

Taking reference to FIG. 13, shown therein is an embodiment of a delayedcorrelation process 1300 that may be effectuated at a receiver inaccordance with the teachings hereinabove. At block 1300, a slidingwindow (e.g., K symbols) spanning a known number of SYNCP and SYNCMsymbols of a preamble sequence is selected. At block 1306, the slidingwindow is placed over the received PLC signal and moved one sample at atime for calculating a delayed correlation for the symbols for eachwindow placement using a select correlation order (N_(C)). That is,within each window placement, each symbol is correlated with itspreceding N_(C) symbols, preferably per samples. Thereafter, absolutevalues of delayed correlations are added together to obtain a totalcorrelation value for each window placement (block 1308), which isrepeated for the window placements over the received stream to obtain acorrelation profile. A predetermined threshold is applied against thecorrelation output profile corresponding to the window placements (block1310). A rough/initial estimate of a preamble start is selected to bethe index of the maximum correlation over a window of length that isdefined by a maximization length parameter that is operative to start atthe index of the first upward (i.e., positive slope) threshold crossing(block 1312).

FIG. 14 illustrates an example cross-correlation scheme for determininga final estimate of a preamble start as part of an embodiment of thepreamble detector of FIG. 8. Example plots of cross-correlation profileswithout and with data peak folding are shown in FIGS. 15A-15Bcorresponding to an embodiment of the cross-correlation scheme of FIG.14. A flowchart of an example cross-correlation method for use with anembodiment of the preamble detector of FIG. 8 is shown in FIG. 16.

As shown at Panel-A 1400A in FIG. 14, the known preamble sequence iscorrelated with a received stream 1402, where the preamble is correlatedwith a window 1404 that slides over the received stream one sample at atime, with a cross-correction (XC) operation 1406 taking between thewindow and received symbols. The correlation output is shown in Panel-B1400B to have 9 peaks located at the start indexes of the 8 SYNCP andthe SYNCM symbols. A threshold value 1410 is applied over the crosscorrelation profile and the maximum correlation is calculated orotherwise determined over a window that starts at the first upwardthreshold crossing 1412 of a length that is determined by themaximization length parameter 1414. It should be appreciated that themaximum peak selected or otherwise previously determined in theforegoing operation corresponds to one of the 9 symbols. In order todetermine the symbol index associated with the maximum detected peak, inPanel-C 1400C, a symbol interval 1416 of a specified length (e.g., aninterval comprising 9 symbols) around the maximum peak for each indexassumption is folded by adding the subintervals of one symbol lengtheach are added together (i.e., folded) to obtain a total of 9folded-correlation windows that correspond to the 9 indexes. Thereafter,a maximization operation is performed over the respective maxima of the9 folded-correlation windows to detect the most likely window index.Within the selected window, the index with the maximum correlation isselected and properly translated to obtain a final estimate for thepreamble start, given the maximum window index. Examples for the crosscorrelation profiles before and after folding are shown in FIGS. 15A and15B at reference numerals 1500A and 1500B, where the X-axis refers tosample index and the Y-axis refers to cross-correlation and combinedcross-correlation values, respectively. In FIG. 15B. the number ofpossible indexes for the maximum peak is from 1 to 8 only as, in thiscase, the index of the maximum peak has only 7 previous symbols referredfrom the receiver start index. In such a scenario, there is nopossibility that the maximum peak corresponds to the 9-th symbol. Asshown in FIG. 15B, the maximum folded-window 1502 corresponds to the8-th symbol and hence the preamble start would be the index of themaximum peak within the 8-th folded window.

FIG. 16 depicts an embodiment of a cross-correlation process 1600 thatmay be effectuated at a receiver in accordance with the teachingshereinabove. At block 1602, a received signal stream is cross-correlatedwith a known preamble sequence (e.g., a legacy preamble), by using asliding window that moves over the received signal stream a fixed numberof samples at a time (e.g., one sample at a time), which sliding windowmay be determined responsive to information from a delayed correlatorincluding an initial estimate and a search range. A cross-correlationoutput profile is obtained (block 1604) that includes peaks at startindexes of known number (K) of SYNCP and SYNCM symbols (e.g., a total of9 symbols comprising 8 SYNCP symbols and 1 SYNCM symbol). Apredetermined threshold is applied over the correlation output profileand a maximum correlation value (i.e., peak) is determined over a windowthat starts at the first upward threshold crossing of a length that isdetermined by a maximum length parameter (block 1606). A foldingoperation is then performed wherein, for a select interval of symbolsaround the maximum peak for each index assumed to be the maximum,subintervals of a specific length (e.g., one symbol length each) areadded together (block 1608), whereby a plurality of folded-correlationwindows (e.g., K windows) are obtained (block 1610). A maximizationoperation is performed to detect the most likely window index (block1612). Within the detected window, the index with the maximumcorrelation is selected and properly translated to obtain a finalestimate of the preamble start of the received signal stream indicatingthe start of a MIMO frame (block 1614).

One skilled in the art will recognize upon reference hereto that apotential detection error that can occur in a preamble detection processis the (erroneous) decision of the start of the second SYNCP as thepreamble start (instead of the first SYNCP), especially when a severechannel condition (e.g., noise) hits one symbol within the preamble. Forinstance, consider scenario 1700A depicted in FIG. 17A, where the secondSYNCP symbol is detected as the preamble's start symbol, indicating anerroneous preamble window 1704 rather than the correct preamble window1702. In order to resolve this boundary error issue, embodiments hereinexploit that the 9-th symbol that corresponds to the full SYNCM symbolhas a negative correlation, as shown at reference numeral 1706. Inaddition, the 10-th symbol also has a negative correlation, with halfthe power, that results from the correlation of the half SYNCM symbolwith known preamble sequence, as shown at reference numeral 1708.Accordingly, in a boundary error scenario where the preamble boundary isshifted by one symbol, subtracting the symbols separated by the 8 (the9-th symbol) and 9 (the 10-th symbol) symbol intervals from the preamblestart symbol multiplies the correlation of the preamble start symbol bya certain amount under ideal conditions. Likewise, performing similarsubtraction operations starting with the incorrect second symbol alsoyields a correlation having a multiplicative factor under idealconditions. For example, in the first case, the correlation of thepreamble start symbol may be 2.5× whereas the correlation in the lattercase may be 1.5× in ideal conditions. However, if a symbol boundaryerror happens in a case where a severe channel hits the first symbol,the result of the comparison between the two correlation values mightstill come in favor of the latter value. Denoting the two correlationvalues as X and Y, respectively, obtained after correspondingsubtraction operations, it can be seen that in the case of correctdetection, the first symbol should be preceded by an inactivity period,and consequently the ratio X/Y should be very small. Hence, if the ratioX/Y is less than one, but still has a considerable value that is greaterthan a certain threshold, then such a case may correspond to a falsedetection of the second SYNCP symbol as the preamble start. To resolvethis issue, embodiment 1700B shown in FIG. 17B provides a thresholdcomparator 1730 that can be optimized via simulations, which takes theoutput of a ratio determination block 1728 that computes a ratio betweenx and Y correction values. Correlations of respective preamble symbolstart conditions, i.e., X value 1724 and Y value 1726, corresponding to1^(st) symbol and 2^(nd) symbol starts, respectively, are determinedresponsive to respective symbol subtraction operations 1720, 1722.Optimized threshold comparator 1730 is configured to provide adetermination to a boundary estimator 1732 wherein if the ratio isgreater than the threshold then the decision would be in favor ofcorrecting the initial preamble start by subtracting one symbol length.Otherwise, the initial estimate of the preamble start is kept unchanged.It should be apparent that similar boundary correction operations mayalso be performed with respect to other symbols of a preamble that arefalsely detected as the preamble start, depending on where the peaks ofpositive and negative correlations occur and their relativedistances/positions.

FIG. 18 is flowchart of a method of symbol boundary correction 1800according to an embodiment of the scheme of FIGS. 17A and 17B. At block1802, a particular symbol (e.g., the 2^(nd) SYNCP symbol) of a preambleis detected as the preamble's start in a received PLC signal(potentially erroneously due to severe channel conditions). Suitablesymbol subtraction operations are performed based on the correct andincorrect symbols (e.g., 1^(st) and 2^(nd) SYNCP symbols, respectively)as the starting symbols and correlation values corresponding torespective preamble start symbols are obtained (block 1804). A ratio ofthe respective correlation values is determined (block 1804), againstwhich a threshold is applied to estimate the correct start symbol (i.e.,the boundary) of the preamble in a received signal (block 1806).

As one skilled in the art will recognize, various embodiments relatingto MIMO frame generation and preamble structure employ CS diversity inthe transmission of the preamble and payload data to provide, interalia, backward compatibility for legacy SISO NB-PLC devices.Furthermore, CS may be advantageously introduced in the transmission ofthe MIMO frames wherein different CS amounts are applied to L-Preamblesand/or M-Preambles transmitted from different phases in order to avoidunintended beamforming that can result in null reception at receiversdue to destructive combining. It should be recognized, however, that theCS vector selected over the transmit phases may also result indestructive combining for some channel configurations. Accordingly,further embodiments herein provide a system where a preamble portion(i.e., the L-Preamble and/or M-Preamble portions) is transmitted usingan initial default CS vector and the communication path is monitored forreception of an acknowledgement signal (e.g., an ACK frame) from thereceiver. If a timeout occurs without receiving an ACK, the transmitteris configured to retransmit the signal with either a zero CS vector oran interleaved version of the initial CS vector or some other variationof the CS vector that may help mitigate the channel conditions whilestill maintaining the desired diversity. FIGS. 19A-19B illustrate a CSselection scheme that may be implemented in a MIMO transmitter (e.g.,transmitter 600A) in an example MIMO PLC network according to anembodiment. Reference numeral 1900A refers to a MIMO channelconfiguration with a transmitter 1902 operative to effectuate threetransmit phases 1906A-1906C that are received at two ports 1908A and1908C of a receiver 1904. The initial CS vector is shown as [0; −X; −2X]wherein CS amounts [0]; [−X] and [−2X] are applied to the three transmitphases 1906A-1906C, respectively. Upon failing to receive an ACK andhaving timed out, transmitter 1902 interleaves or otherwise rearrangesthe CS vector as [−2X; −X; 0] applied to the transmit phases1906A-1906C, respectively, in order to provide a different channel forreceiver 1904, as illustrated in FIG. 19B.

FIG. 20 is flowchart of a CS selection method operative corresponding toan embodiment of the scheme of FIGS. 19A-19B. At a MIMO transmitter, aninitial/default CS vector (e.g., having a dimension of N_(T), the numberof transmit ports or phases) is selected or otherwise configured (block2002). The initial/default CS vector is applied to a preamble portion(e.g., L-Preamble and/or M-Preamble portions), whereupon a MIMO signalframe comprising the CS-applied preamble portions or at least a portionthereof is transmitted to receive device(s), as set forth at block 2004.A determination is made whether an ACK is received within a timeoutperiod (block 2006). If so, the transmitter continues with the currenttransmit transaction using the selected CS vector (block 2008). On theother hand, if no ACK is received within a timeout period, the CS vectorvalues are interleaved or otherwise rearranged, which are applied to thepreamble sections that are retransmitted. The CSinterleaving/rearrangement process may take place iteratively until anACK is received and a CS vector is selected in the iterative process(block 2010). Thereafter, the selected CS vector that has been suitablyrearranged is applied or continued for the (re)transmission (block2012).

It should be realized that when MIMO devices as well as legacy SISOdevices are disposed in a mixed-mode PLC network implementation, thecollision rate between the MIMO devices and legacy devices should beminimized as much as possible while still allowing access to thephysical medium using a standardized technique such as CSMA.Accordingly, one of the goals of backward compatibility of embodimentsof a MIMO frame structure with the legacy frame structure is to enablethe legacy devices to determine whether the channel is busy or idle.During a MIMO packet transmission, if the legacy device is able todetect the L-Preamble and decode the FCH, it will be able to correctlydetermine the length of the MIMO packet and will back off for the timeof the MIMO packet transmission. On the other hand, if the legacy devicewas able to detect the L-Preamble, but was not able to decode the FCHcorrectly, a CRC failure would occur. Hence, in order to reduce the rateof possible collisions between the MIMO and the legacy PLC devices,intentional errors can be inserted into the legacy part of the FCHwithin the transmitted frame from the MIMO PLC device, which ensures CRCfailure at the legacy PLC device, thereby leading to a maximum back-offlength instead of having a random back-off period. In one embodiment,the back-off time/length may be preconfigured, predetermined orotherwise provided specific to a particular implementation.

FIG. 21 is flowchart of a collision rate reduction method 2100 accordingto an embodiment that may be implemented as part of a PLC device, e.g.,transmitter 600A shown in FIG. 6A. At block 2102, the transmitter isoperative to obtain or otherwise generate legacy FCH data and compute aCRC sequence therefor. As described previously, the legacy FCH datapopulates an FCH portion of a MIMO data signal frame that includesMIMO-compliant FCH data, a M-Preamble portion and a payload dataportion. A perturbation is introduced in at least a portion of thelegacy FCH data in order to insert one or more intentional errors piecesof the legacy FCH data (block 2104). In one implementation, the legacyFCH data may be perturbed so as to maximize a Hamming distance betweenthe perturbed and unperturbed FCH data. In other implementations, thelegacy FCH data may be disturbed by inverting all or a portion of thebits, adding extra bits, deleting one or more bits, and the like. Atblock 2106, the MIMO data signal frame that includes the perturbedlegacy FCH data and the CRC sequence of unperturbed legacy FCH data isencoded and transmitted, wherein the CRC sequence is configured to failat a legacy PLC receiver device in order to ensure a predeterminedback-off time by the legacy PLC receiver device when it receives theMIMO data signal frame via the PLC network. It should be apparent thatthe foregoing methodology requires only inserting errors in thetransmitted frame from the MIMO PLC device and does not requiresubstantial changes in either the transmitter or the receiver portion ofa legacy PLC device.

FIG. 22 is a block diagram of an example PLC device 2200 wherein one ormore embodiments of the present patent application may be practiced. PLCdevice 2200 comprises a modem 2214 that may include parts of at leastsome of the transmit/receive embodiments described hereinabove, e.g.,FIGS. 4A-4B and/or FIGS. 6A-6B. Although not specifically shown, itshould be appreciated that modem 2214 may contain one or moreprocessors, including digital signal processors (DSPs), associatedmemory, and other circuitry in realizing one or more aspects set forthin the present disclosure. Also, one or more processors 2210 havingassociated memory 2208 and timers 2212 may be provided for operating inconcert with modem 2214 in a data communications environment, wherebyPLC device 2200 may be configured to operate as a data communicationsdevice, e.g., a desktop computer, laptop computer, cellular phone, smartphone, or as a service node, data concentrator node, etc. Suitable AFE2204 allows coupling of the device 2200 to a power line installation2202, which may be part of example installation 102 shown in FIG. 1.

One of ordinary skill in the art will understand that the components ofthe power line communications systems described herein may be embodiedas individual circuits or separate components, or as a single devicethat performs more than one of the illustrated operations. For example,in one embodiment, the transmitters and receivers described herein maybe embodied as a microprocessor, central processing unit (CPU),integrated circuit (I/C), or application specific integrated circuit(ASIC), in conjunction with other circuitry. Software, firmware, orother embedded instructions may control the operation of the transmitterand receivers and cause the component to perform the functions describedherein. Such devices may further perform the transmitter/receiverprocessing and MIMO frame signal generation, precoding, receiverprocessing and signal generation, power line coupling, signal combining,diversity distribution, channel or transmission matrix construction, S/Pand P/S conversion, IFFT and FFT processing, CP addition and removal,OFDM (de)coding, CS selection and/or (re)configuration, and preambledetection, etc.

In the above-description of various embodiments of the presentdisclosure, it is to be understood that the terminology used herein isfor the purpose of describing particular embodiments only and is notintended to be limiting of the invention. Unless otherwise defined, allterms (including technical and scientific terms) used herein have thesame meaning as commonly understood by one of ordinary skill in the artto which this invention belongs. It will be further understood thatterms, such as those defined in commonly used dictionaries, should beinterpreted as having a meaning that is consistent with their meaning inthe context of this specification and the relevant art and may not beinterpreted in an idealized or overly formal sense expressly so definedherein.

Furthermore, in the above description, at least some terminology is usedthat is specifically defined in the NB-PLC standards such IEEE 1901.2,G2, etc. and/or is well understood by those of ordinary skill in the artin PLC technology. Definitions of these terms are not provided in theinterest of brevity. Additionally, this terminology is used forconvenience of explanation and should not be considered as limitingembodiments of the invention to the IEEE 1901.2 standard. One ofordinary skill in the art will appreciate that one or more of thedisclosed embodiments may be practiced in conjunction with otherstandards, mutatis mutandis, without departing from the describedfunctionality.

At least some example embodiments are described herein with reference toblock diagrams and/or flowchart illustrations of computer-implementedmethods, apparatus (systems and/or devices) and/or computer programproducts. It is understood that a block of the block diagrams and/orflowchart illustrations, and combinations of blocks in the blockdiagrams and/or flowchart illustrations, can be implemented by computerprogram instructions that are performed by one or more computercircuits. Such computer program instructions may be provided to aprocessor circuit of a general purpose computer circuit, special purposecomputer circuit, and/or other programmable data processing circuit toproduce a machine, so that the instructions, which execute via theprocessor of the computer and/or other programmable data processingapparatus, transform and control transistors, values stored in memorylocations, and other hardware components within such circuitry toimplement the functions/acts specified in the block diagrams and/orflowchart block or blocks, and thereby create means (functionality)and/or structure for implementing the functions/acts specified in theblock diagrams and/or flowchart block(s). Additionally, the computerprogram instructions may also be stored in a non-transitory tangiblecomputer-readable medium that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablemedium produce an article of manufacture including instructions whichimplement the functions/acts specified in the block diagrams and/orflowchart block or blocks.

Still further, in at least some additional or alternativeimplementations, the functions/acts described in the blocks may occurout of the order shown in the flowcharts. For example, two blocks shownin succession may be executed substantially concurrently or the blocksmay sometimes be executed in the reverse order, depending upon thefunctionality/acts involved. Moreover, the functionality of a givenblock of the flowcharts and/or block diagrams may be separated intomultiple blocks and/or the functionality of two or more blocks of theflowcharts and/or block diagrams may be at least partially integrated.Furthermore, although some of the diagrams include arrows oncommunication paths to show a primary direction of communication, it isto be understood that communication may occur in the opposite directionrelative to the depicted arrows. Finally, other blocks may beadded/inserted between the blocks that are illustrated.

It should therefore be understood that the order or sequence of theacts, steps, functions, components or blocks illustrated in any of theflowcharts depicted in the drawing Figures of the present disclosure maybe modified, altered, replaced, customized or otherwise rearrangedwithin a particular flowchart or block diagram, including deletion oromission of a particular act, step, function, component or block.Moreover, the acts, steps, functions, components or blocks illustratedin a particular flowchart may be inter-mixed or otherwise inter-arrangedor rearranged with the acts, steps, functions, components or blocksillustrated in another flowchart and/or block diagram in order toeffectuate additional variations, modifications and configurations withrespect to one or more processes for purposes of practicing theteachings of the present patent disclosure.

Although various embodiments have been shown and described in detail,the claims are not limited to any particular embodiment or example. Noneof the above Detailed Description should be read as implying that anyparticular component, element, step, act, or function is essential suchthat it must be included in the scope of the claims. Reference to anelement in the singular is not intended to mean “one and only one”unless explicitly so stated, but rather “one or more.” All structuraland functional equivalents to the elements of the above-describedembodiments that are known to those of ordinary skill in the art areexpressly incorporated herein by reference and are intended to beencompassed by the present claims. Accordingly, those skilled in the artwill recognize that the exemplary embodiments described herein can bepracticed with various modifications and alterations within the spiritand scope of the claims appended below.

What is claimed is:
 1. A detector comprising: an input port configured to receive a PLC signal stream over a first pair of power lines of a power line communications (PLC) network, wherein the received PLC signal stream comprises a preamble; a first detector configured to determine an initial estimate of a preamble start for the preamble in the received PLC signal stream by correlating the received PLC signal stream with a delayed version of the PLC signal stream; and a second detector configured to: correlate the received PLC stream with a known preamble to generate an cross-correlation output profile; and determine a final estimate of the preamble start based in part on the cross-correlation output profile and the initial estimate of the preamble start; wherein the preamble has a known number K of symbols, K is an integer greater than or equal to 1, and the first detector is configured to perform the correlating action by: placing a window of length equal to K symbols and divided into K symbol regions over the received PLC signal; and adding one sample in the PLC signal stream to the end of the window and remove one sample in the PLC signal stream from the beginning of the window.
 2. The detector of claim 1, wherein the known preamble is a legacy preamble according to a legacy PLC data transmission standard or a Multi-Input Multi-Output (MIMO) preamble.
 3. The detector of claim 1, wherein the input port is configured to receive a file control header (FCH).
 4. The detector of claim 1, wherein the received PLC signal stream comprises a Multi-Input Multi-Output (MIMO) frame received via an [N_(T)×N_(R)] MIMO channel in the PLC network, the MIMO frame including a legacy preamble portion, a Frame Control Header (FCH) portion including legacy FCH data and MIMO-compliant data, a MIMO-compliant preamble portion, and a payload data portion.
 5. A method comprising: receiving a power line communication (PLC) signal stream over a first pair of power lines of a PLC network, wherein the received PLC signal stream comprises a preamble; determining an initial estimate of a preamble start for the preamble in the received PLC signal stream by correlating the received PLC signal stream with a delayed version of the PLC signal stream; correlating the received PLC stream with a known preamble to generate an cross-correlation output profile; and determining a final estimate of the preamble start based in part on the cross-correlation output profile and the initial estimate of the preamble start; wherein the preamble has a known number K of symbols, K is an integer greater than or equal to 1, and determining the initial estimate of the preamble start by correlating includes: placing a window of length equal to K symbols and divided into K symbol regions over the received signal; and adding one sample in the PLC signal stream to the end of the window and removing one sample in the PLC signal stream from the beginning of the window.
 6. The method of claim 5, wherein the preamble is a first Multi-Input Multi-Output (MIMO) preamble.
 7. The method of claim 5, wherein the received PLC signal stream comprises a Multi-Input Multi-Output (MIMO) frame received via an [N_(T)×N_(R)] MIMO channel in the PLC network, the MIMO frame including a legacy preamble portion, a Frame Control Header (FCH) portion including legacy FCH data and MIMO-compliant data, a MIMO-compliant preamble portion, and a payload data portion.
 8. The method of claim 5, wherein the preamble is used to provide channel estimation for decoding a file control header. 